Low Energy Energy Arc Joining Process Process for Materials Materials Sensi Sensi tiv e to Heat Heat S. F. Goecke, EWM Mündersbach, Germany
Modern, ultra-lightweight places demands on welding technology that simply cannot be met with traditional shielding gas welding processes. Variants of the robust arc welding process need to be developed which feed very coldArc is a variant of the MIG/MAG little heat into the material but which still guarantee strong connections. The coldArc is process that meets these demands. In this process, all interventions in the process flow are carried out directly in the power source without mechanical intervention in the wire feed, which means that standard welding torches can be used and the process can also be used to produce excellent manual welding results. 1
After a specific arc burning duration, a drop forms on the tip of the electrode which, as the arc is relatively short, quickly comes into contact with the molten pool, and the arc goes out. The surface tension of the pool draws the drop away from the electrode tip – in the final phase of the separation process if the bridge has already been constricted, the pinch effect also contributes to this via the Lorentz Force as well as the Joule heating effect from the rapidly increasing current density – and after the liquid bridge between the electrode and the workpiece breaks, the arc re-ignites. What happens in terms of the electrical forces is also shown in Figure 1. 1. At the start of the short circuit, the voltage falls because the electrical resistance of the liquid bridge is now lower than the previous resistance level in the arc. At the same time, the current starts to increase to the value of the short circuit current. As soon as the bridge between the electrode and the workpiece breaks, the voltage then increases very quickly as there is an increase in voltage required to ignite the arc. The voltage fall starting at that point is very slow, however, due to the inductivities in the welding current circuit, the re-ignition process takes place under a relatively high electrical output. In this process, part of the liquid bridge can evaporate in an explosive way if not counteracted by sufficient choke effect in the current circuit in advance of the increase in current. The consequence is either significant spatter formation or a very low process dynamic through to instability. For welding tasks requiring low heat effects, e.g. when welding very thin metal sheets with poor fit up, it is much more damaging because the weld metal drops through at the re-ignition point, creating a hole. When welding metal sheets with surface finishes, e.g. zincplated sheets, there is also a risk of the surface coating evaporating and burning away next to the joint and on the reverse side. With higher strength steels, softening can occur if the heat feeding is too great. The normal short arc, otherwise an excellent tool for welding thin sheets, is therefore not suitable for these types of welding tasks which are extremely sensitive to heat.
Introduction
In addition to the concepts of "higher, further, faster" which have represented the challenges to the modern world of technology for many years, recently a demand for "easier" has also come to the fore. This applies primarily in vehicle construction where fuel can be saved during acceleration, driving and when braking by reducing weight, which in turn preserves resources, reduces costs and protects the environment. This more recent demand has produced increasingly lightweight models which are only made possible by the use of thinner high-strength steel metal sheets, normally plated, and lighter materials such as aluminium and magnesium. However, this type of lightweight design places demands on welding technology which simply cannot be met using standard welding machines. This meant it was necessary to develop new processes that expose the joint to an extremely low level of heat during the welding process. The coldArc is coldArc is just such a process. 2
The short arc; the conventional method for low-energy low-energy welding
The short arc is used in MIG/MAG welding in the lower power range, i.e. at lower currents and lower voltages. In this process a form of material transfer is used which features cyclical, repeating arc phases and short circuit phases, Figure 1. 1. Phase 1 Arc b urn ing
Phase 2 Short-circuit
Phase 3 Short-circuit resolution Short-circuit and renewed burning phase
Us
t Is t
Figure 1 Material transfer (schematic), current and voltage outlines in short arc welding. © 2005 EWM HIGHTEC HIGHTEC WELDING GmbH GmbH
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Phase 1 Arc bu rni ng
Approaches to improvin g the short arc process
There have been no shortage of attempts to improve the behaviour of the short arc, especially on re-ignition after the short circuit, and to use a short arc with reduced heat feeding. As early as the 1980s, attempts were made to reduce the current immediately before the short circuit bridge breaks and then to provide a high voltage pulse to ease the re-ignition process. This did reduce the spatter formation, but the heat feeding was only slightly reduced, [1] and [2]. Further steps down this path were the modified short arc ChopArc, [3] and [4], which as a process-safe MAG welding process achieved considerable progress, especially in the minimum thickness sheet range 0.8 0.2 mm. In addition, an adaptive control system was developed here which optimised the process quality in real-time, [5]. More recent developments have worked with a discontinuous wire feed, i.e. the duration of the short circuit is reduced so that the wire electrode is retracted slightly during the short circuit so that the short circuit bridge breaks more easily. This means that a lower energy welding process with low spatter formation can be achieved, [6]. Because a push-pull drive with two wire feed motors with high dynamics is required, this process is more suitable for automated welding and is only used in combination with welding robots. 4
Phase 3 Short-circuit resolution and renewed burning phase
Us
t Is t
Figure 2 Material transfer (schematic), current and voltage outline in the coldArc process. It is used as a guideline value when controlling the current. However, the continuous measurement of the voltage with the corresponding reaction to all changes in voltage is required to achieve this (highly dynamic instantaneous value regulation). A digital signal processor (DSP) can then be used to extract the power from the arc immediately before re-ignition in a period of less than 1 µs, Figure 2, so that the reignition takes place very gently. So that a sufficient quantity of molten material can be formed immediately on the electrode tip however, there is an increase in the amount of energy required. Immediately after the arc re-ignites, the current is therefore raised back up again for a defined short period to what is known as the melt pulse. Only then is the current lowered to an extremely low basic level to minimise further melting, and the next cycle begins. This melt pulse after each short circuit generates a melting cone of a constant size on the electrode which means that process continues very smoothly and evenly. This is the only way it has been possible to work at extremely low currents in the phases between the short circuits, without the wire melting further or the arc going out. All this goes to make up the very low-energy coldArc process. Figure 3 shows a sequence of images from a highspeed film, which highlight the very even material transfer and the gentle ignition of the arc.
coldArc –
successful joi ning in tasks demanding low heat
Development work with the aim of achieving a lowenergy process without mechanical intervention in the wire feed process, resulted in a process variant in which all necessary interventions in the process take place in the power source alone. This variant of the MIG/MAG process, known as coldArc, is also a short arc process, and is called such due to the cyclical change between the arc and short circuit phases. As the electrical output during the re-ignition process is a critical criterion for successfully welding thin sheets, active intervention is carried out in the outline of the power intake for the overall process, however, in other words during the arc phase, in the short circuit phase and especially when re-igniting the arc, Figure 2, the voltage outline remains the same as in the normal short arc process.
© 2005 EWM HIGHTEC WELDING GmbH
Phase 2 Short-circuit
5.
What the
coldArc process
can do
The outline of the arc output on arc re-ignition is shown in Figure 4. The advantages of the coldArc process in comparison to the standard short arc at the moment of re-ignition and immediately afterwards become very clear. Here the output at the moment of arc re-ignition is considerably lower not just as an absolute value.
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Figure 3
Sequence of the material transfer in the coldArc process taken from high-speed pictures, 8,000 B/s.
In fact, immediately after the arc ignites, the output is reduced in an exceptionally dynamic and controlled way, and then, after the arc has been stabilised, increased to the defined melting of the electrode tip in a pulsed way. A process of this type can be used for many welding tasks, especially in vehicle construction where the normal short arc is no longer suitable. Even just a few years ago, it was assumed that the MIG/MAG process should be used for steel over a panel thickness of 0.7 mm and for aluminium over 3 mm [7]. The panel thicknesses in vehicle construction today are becoming increasingly thin, however. They already go down to as low as 0.3 mm, and 0.2 mm is already being tested for composite construction work. The difficulties in achieving an even groove are even greater if there are larger air gaps to be bridged. This is a typical task for the coldArc process.
The copper-based wires have a melting point of around 1000 °C. In comparison to the same type of MAG welding, the heat loading of the surface layers is already therefore considerably reduced. These are even more protected if MIG brazing is carried out using zinc-based solder with melting intervals of around just 450 °C. The use of these wires is only possible however, if the short circuit current is strictly limited and the general heating feeding is also reduced considerably. The vaporisation temperature of the zinc/aluminium alloys used for arc brazing is just under 900 °C, in other words still below the melting temperature of copper alloys.
Standard short arc
PS
cold Arc arc
Figure 5 Manual coldArc brazed joint of 0.8 mm
electrolytically plated steel plate with 4.0 mm air gap, 1.0 mm CuSi3 wire.
Output on re-ignition
Figure 4 Minimised arc output of the coldArc process
on re-ignition. For some time now, different welding techniques have been used on surface-coated metal sheets, in other words, using copper-based filler material for arc brazing. This helps to preserve the zinc layer, but difficulties can arise if there is a larger air gap. With the coldArc process, on the other hand, even larger air gaps can be bridged with the filler material. Figure 5 shows 0.8 mm thick zinc-plated steel metal sheets which have been brazed manually with air gaps of as large as 4 mm in the vertical down position using 1.0 mm CuSi3 wire with a moderate current of 50 A and a voltage of 13.5 V coldArc. © 2005 EWM HIGHTEC WELDING GmbH
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It is not possible to fusion-weld these two materials directly because inter-metallic Al/Fe phases form which are exceptionally brittle, Figure 8.
Figure 6 Electrolytically galvanised steel metal
sheets, fillet joint on the lap joint with zinc wire brazed using the coldArc process. On re-ignition, the short circuit bridge would therefore vaporise immediately in an explosive way and blow away the lightweight welded material if the short circuit current is not reduced considerably. With the coldArc process, MIG brazing with zincbased wires is possible for the first time without restrictions. Figure 6 shows the surface and the reverse side of a lap joint on 0.75 mm thick galvanised metal sheets which have been joined using this low-melting wire. The zinc layer is completely preserved, both immediately next to the groove and on the reverse side. In the brazing process it would have become completely liquid, but it would not have vaporised. In vehicle construction work, mixed joints between steel and aluminium are also increasingly being used.
Figure 8 Phase diagram for iron/aluminium.
The phase diagram shows that iron or steel and aluminium offer virtually no solubility with one another. In each mixed ratio, Fe/Al phases occur which feature brittle characteristics. Experience therefore shows that a proportion of Al/Fe phases in the molten material of over 10% must be avoided in all cases. When using zinc as the filler material, a joint can then be created between these two materials, where the aluminium is partially melted, whereas the steel, to avoid brittleness in the molten material, may only be moistened. This means that a welded joint is created on one side, and a brazed joint on the other. Figure 7 shows an overview picture and a detailed picture of this type of joint, brazed using the coldArc process with a zinc-based wire, and an application from vehicle body construction. The strength values achieved with zinc wires in the fillet weld on the lap joint are in the range of the strengths of aluminium wrought alloys and of MIG brazed joints using copperbased wire. With butt joints, slightly lower strength values are achieved. Even here the use of push/pull torches is not required; completely normal MIG/MAG welding torches can be used for coldArc welding and coldArc brazing. Other typical applications for coldArc brazing and coldArc welding are shown in Figure 9 to Figure 13.
Figure 7 Mixed aluminium/steel joints with zinc-based
wires; Top: Overview picture Bottom left: Detailed picture Bottom right: Car door © 2005 EWM HIGHTEC WELDING GmbH
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Literature
[1] Kabushiki Kaisha Kobe Seiko Sho: Output control of SC welding power source, PatNr.: US 4546234, Kobe Steel, Japan, 1984 Figure 9
Hot dipped steel metal sheets, 0.7 mm, fillet on the lap joint, with 1.0 mm Zn wire coldArc brazed with 0.35 m/min, U=13.5 V, I=40 A.
[2] The Lincoln Electric company: STT – Surface Tension Transfer , Pat.Nr.: EP 0324960 B1, USA, 1989, and EP 1232825 A3, USA, 2002 [3] Goecke, S. F. and L. Dorn: Research on the influence of process control and shielding gas composition on spatter formation and groove geometry in MAG short arc welding of thin steel sheets less than 0.5 mm thick, Final Report DFG (Deutsche Forschungsgemeinschaft; German Research Association) Do 202/26-3, (2000)
Figure 10
[4] Goecke, S. F., L. Dorn and M. Hübner: MAG ChopArc welding for minimum-thickness sheets ≥ 0.2 mm. Individual conference report in the conference volume for the Great Welding Conference – GST 2000, Nürnberg, 27-29 Sep. 2000, German Welding Association Reports, Volume 209, (2000), pages 163-168
Al-St mixed joint, 0.7 mm hot dip steel and 1.0 mm AlMg, fillet weld on the lap joint, with 1.0 mm Zn wire coldArc brazed at 0.35 m/min, U=13.5 V, I=40 A.
[5] S. F. Goecke, E Metzke, A Spille-Kohoff, M Langula: ChopArc – MSG arc welding for ultralightweight construction, bmb+f-gefördertes Verbundprojekt, final report, Fraunhofer IRB Verlag, 2005, ISBN 3-8167-6766-4 Figure 11
[6] G Huismann: Direct control of the material transfer, the Controlled Short Circuiting (CSC)MIG process, ICAWT 2000: Gas Metal Arc Welding for the 21st Century, Dec. 6-8, 2000, Orlando, Florida
Al-St mixed joint, 1.0 mm AlMg and 0.7 mm hot-dip steel, fillet weld on the lap joint, with 1.0 mm AlSi5 wire coldArc brazed at 1.1 m/min, U=14.5 V, I=60 A.
[7] R Killing: Handbook of Welding Techniques, Part 1 – Arc welding, specialist book series on welding technology, volume 76/I, DVS-Verlag Düsseldorf 1999
Figure 12
Steel sheet, 1.0 mm, butt joint, 1 mm gap, 1.0 mm G4Si1 wire, coldArc welded at 2.0 m/min, U=19 V, I=136 A.
Figure 13
CrNi sheet, 0.5 mm, fillet weld on lap joint, 0.8 mm wire, coldArc welded at 2.0 m/min, U=16.5 V, I=90 A.
© 2005 EWM HIGHTEC WELDING GmbH
[8] S. Kröger and R. Killing: Software for creating and managing parameters for MIG/MAG welding , German Welding Association Yearbook 2004, pages 150-161, DVS-Verlag, Düsseldorf 2003
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